Add local error handling in WebClient.retrieve

This commit introduces a way to customize the WebClientExceptions, as
thrown by WebClient.ResponseSpec.bodyTo[Mono|Flux]. The first
customization will override the defaults, additional customizations are
simply tried in order.

Issue: SPR-15724

spring-web/src/main/java/org/springframework/http/HttpStatus.java

@@ -465,6 +465,16 @@ public enum HttpStatus {
 		return Series.SERVER_ERROR.equals(series());
 	}
 
+	/**
+	 * Whether this status code is in the HTTP series
+	 * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR} or
+	 * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
+	 * This is a shortcut for checking the value of {@link #series()}.
+	 */
+	public boolean isError() {
+		return is4xxClientError() || is5xxServerError();
+	}
+
 	/**
 	 * Returns the HTTP status series of this status code.
 	 * @see HttpStatus.Series